When I moved to New York in the mid-90s and witnessed for the first time how Americans celebrate St Patrick's Day I was kind of blown away. Growing up in Ireland the day that has become synonymous with Irishness around the world was a low-key event, which involved little more than wearing a bit of shamrock and getting a day off school. In recent years it has become a much bigger deal back home with Mardi Gras style celebrations happening in all the major cities but we still can't rival the sheer magnitude of the occasion as it is celebrated in America by Irish and non-Irish alike.

It might simply be the happy coincidence that the anniversary of the death of St Patrick falls around the beginning of Spring, a time when we could all use a party. Or the fact that Americans love to celebrate full stop and do it better than most. As Malachy McCourt, the much loved New Yorker/Limerick man put it to me: "It's a great credit to Americans that they want to get exuberant at the slightest opportunity." But when year after year millions of people choose to be, as the saying goes, "Irish for the day", there must be more to it than an excuse to drink green beer.

The St Pat's for All parade in Queens (which was originally founded to accommodate Irish lesbians and gays who are to this day prohibited from participating in the official parade on Fifth Avenue) opened this year with a blessing by the Indians of the Choctaw Nation followed by a rap by a young Tibetan refugee about his own immigrant experience.

The parade itself featured a Bolivian dance troupe, a group of Chileans who have marched every year since 2000 and the Mexican community who marched under the banner honouring Our Lady of Guadalupe.

There was a healthy smattering of Irish people in attendance too of course, no shortage of ceol agus craic (music and good times) and tricolours and shamrocks were the prevailing symbols of the day. But this unlikely gathering of diverse groups seemed to me to be more about the common struggle all immigrants face of trying to hold on to their heritage while they forge new identities as Americans, than about any nebulous genealogical connection to Ireland, though those connections do exist.

Because of the famine and subsequent emigration, Ireland has lost almost half its population since 1840. And as Niall O'Dowd, editor and publisher of the Irish Voice Newspaper put it, "those emigrants cast a wide net". The first head of state of an independent Chile was a man called Bernardo O'Higgins and the Bolivian born Ernesto Che Guevara's grandmother was Anna Lynch from Galway. I don't know of any Choctaw Indians with Irish surnames but our long-held fondness for their nation has to do with their extraordinary act of kindness during the famine. (Moved by stories of Irish starvation, the Choctaws sent a generous donation to a famine relief fund.)

But it's not surnames that start with an O' that bring these people together nor is it to celebrate a catholic saint, who offers only a limited version of Irishness to those of us who are atheists or Protestants or subscribers to the numerous other belief systems on offer. What brought all these communities together in Queens and what causes millions of people from all sorts of backgrounds across America to become Irish for the day is the shared bond of being immigrants.

As one of those immigrants, I can attest that leaving your country and your family behind to find a more feasible life in a foreign land is no small decision. And it is one that's made, more often than not, out of necessity rather than choice. As almost every family in America, other than the natives, has someone from some generation who has made that decision, it's no surprise that they can connect so easily with the diaspora of a nation who has lost so much of its population to emigration.

I'd like to think anyway that St Patrick's day is something more than a grand old excuse for a party – not that there's anything wrong with that. But either way, this year again the Irish Taoiseach Brian Cowen who hails from County Offaly will meet up with the great-great-great-grandson of a fellow Offaly man called Fulmouth Kearney, better known outside of Ireland as President Barack Obama, and the business of the White House will be put on hold for the day so that a nation of immigrants can celebrate the legacy of a nation of emigrants.